Mean corpuscular volume as a prognostic factor for 30-day mortality in major trauma patients: a retrospective cohort study

We investigated the clinical implications of the mean corpuscular volume (MCV) in patients with major trauma. This single-center retrospective review included 2021 trauma patients admitted to the intensive care unit between January 2016 and June 2020. We included 1218 patients aged \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge $$\end{document}≥ 18 years with an injury severity score \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge $$\end{document}≥ 16 in the final analysis. The clinical and laboratory variables were compared between macrocytosis (defined as MCV \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge $$\end{document}≥ 100 fL) and non-macrocytosis groups. Cox regression analysis was performed to calculate the hazard ratios (HRs) of variables for 30-day mortality, with adjustment for other potential confounding factors. The initial mean value of MCV was 102.7 fL in the macrocytosis group (n = 199) and 93.7 fL in the non-macrocytosis group (n = 1019). The macrocytosis group showed a significantly higher proportion of initial hypotension, transfusion within 4 and 24 h, and 30-day mortality than the non-macrocytosis group. Age (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge $$\end{document}≥ 65 years), hypotension (systolic blood pressure \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\le $$\end{document}≤ 90 mmHg), transfusion (within 4 h), anemia (Hb < 12 g/day in women, < 13 g/day in men), and macrocytosis were significantly associated with 30-day mortality (adjusted HR = 1.4; 95% confidence interval 1.01–1.94; p = 0.046) in major trauma patients. Thus, initial macrocytosis independently predicted 30-day mortality in patients with major trauma at a Level I trauma center.


Variables studied and definitions
The patient baseline characteristics included age; sex; initial systolic blood pressure (SBP); heart rate (HR); and trauma-related variables, such as the ISS, revised trauma score (RTS), and clinical outcomes such as duration of ICU stay, hospital stay, and the number of days on ventilatory support.Complete blood count (CBC) data was collected from blood samples obtained immediately in the emergency room.
Hypotension was defined as a systolic blood pressure < 90 mmHg.Anemia was diagnosed based on Hb levels < 13 g/dL in men and < 12 g/dL in women 3 .Elderly individuals were defined as those aged ≥ 65 years.An emergency operation was defined as surgeries performed within 24 h after admission to the emergency room.Transfusion within 4 h and 24 h with any blood component were expressed as transfusion_4 and transfusion_24, respectively.Macrocytosis was defined as an MCV value of ≥ 100 fL.

Statistical analyses
Statistical analyses were performed using the R software version 4.1.0(The R Foundation, Vienna, Austria) using additional packages: "moonBook", "survival", "ggplot2", and "coxphf ".Listwise deletion was used for handling missing data.Categorical data are presented as frequencies with proportions and were compared using the chi-square or Fisher's exact test.Continuous variables with normal distribution are expressed as mean and standard deviation, whereas those with skewed distribution are expressed as medians and interquartile range (IQR).The continuous variables were compared between groups using the Student's t-test or Mann-Whitney U test.The initial macrocytosis and time to survival event were calculated using the Kaplan-Meier and log-rank tests, respectively.Statistical significance was set at p < 0.05.The Cox proportional hazards model was used for survival analysis, with adjustment for other potential confounding factors.

Results
The patient baseline characteristics are presented in Table 1.Of the 1218 patients included in the study, initial macrocytosis was observed in 16.3% of patients (199 patients).The mean age was 57.6 years, and the age of patients in the macrocytosis group was significantly higher than that of patients in the non-macrocytosis group (61.2 vs. 56.9,p = 0.001).However, the proportion of elderly patients (aged ≥ 65 years) was not significantly different between the two groups (42.2% vs. 36.0%,p = 0.115).The initial SBP showed a significant difference between groups, and the proportion of hypotension was significantly higher in the macrocytosis group than in the non-macrocytosis group (34.7% vs. 26.6%,p = 0.006).However, ISS showed no significant difference (25.0 vs. 24.8,p = 0.704) between groups, and the AIS of the head and neck, chest, and abdomen also showed no significant difference.
The macrocytosis group underwent more transfusions than did the non-macrocytosis group during the initial 4 h (46.7% vs. 35.0%,p = 0.002) and 24 h (57.8% vs. 48.1%,p = 0.015) after admission.The proportion of emergency operations and the duration of ICU stay showed no significant differences between the two groups.The 30-day mortality rate was 24.1% in the macrocytosis group, which was higher than that in the non-macrocytosis group (14.5%) (Table 2).The 30-day survival curve for the macrocytosis group compared with that for the nonmacrocytosis group is shown in Fig. 1.Both groups had acutely increased mortality for the initial 10 days, after which the mortality curves increased more slowly.
To identify factors associated with 30-day mortality, Cox regression proportional hazards analysis was performed using several binary compound factors such as age, transfusion_4, hypotension, anemia, and macrocytosis (Fig. 2).All of these factors were significantly associated with 30-day mortality in severely injured trauma patients.

Discussion
The primary finding of this study was that initial macrocytosis was associated with 30-day mortality in elderly patients with major trauma, along with factors such as transfusions within 4 h of admission, hypotension, and anemia.
In our cohort, the prevalence of macrocytosis was 16.3%, and it was associated with increased 30-day mortality compared to that in the non-macrocytosis group.This finding may be explained by hypoperfusion due to major trauma in patients with macrocytosis.The relationship between MCV and Hb was established almost a century ago 14 , which was validated in a more recent study where MCV was shown to vary in a strict linear relationship with the average Hb content of RBCs 15 .Accordingly, larger erythrocytes (macrocytes) accommodate a greater amount of Hb compared to smaller erythrocytes 16 .In our study, no statistical differences were found in the mean MCV values between the anemic and non-anemic groups (Supplementary Table S1); however, between the hypotension and non-hypotension groups, the mean MCV value and proportion of macrocytosis showed significant differences (Supplementary Table S2).
Multiple major traumas can cause systemic inflammatory response syndrome (SIRS) via hormonal, metabolic, and immunological mediators 17,18 .This response occurs immediately after major trauma and aggravates the initial damage caused by hypoperfusion and reperfusion 19 .This metabolic response is associated with increased oxygen demand in the tissues 17 .Consequently, to compensate for the increased oxygen consumption, the body reacts with tachycardia, increased cardiac output, increased respiratory rate, and vasodilatation 17,20 .As mentioned previously, anemia due to acute blood loss, hemolysis, or malignancy is known as normocytic anemia 1 .In most cases, the etiology of macrocytosis may involve abnormal RBC development, abnormal RBC membrane composition, increased reticulocyte count, or a combination of these factors 21 .Although the results of the high proportion of   macrocytosis in our study are not explained by any particular etiologies, it seems reasonable that erythrocytes are enlarged to respond to the increased oxygen demand; however, biological studies are required to confirm this.Erythrocytes transport oxygen and carbon dioxide, and their membranes have systemic antioxidant properties since they are potentially exposed to oxidative stress 22 .Macrocytosis is a structural and functional abnormality of the erythrocyte membrane 12,21 .Oxidative stress resulting from the overproduction of reactive oxygen species is considered to contribute to the development and progression of cardiovascular disease 23 .Therefore, MCV is not only an indicator of anemia but also a marker of inflammation and endothelial function 9 .Studies investigating the underlying pathophysiology of macrocytosis in patients with trauma are required in the future.
Our study had several limitations.First, it was a retrospective, observational, single-center study.Second, we could not account for confounding factors such as chronic hepatitis, chronic renal disease, or alcohol abuse on MCV changes, and the influence of these factors should be addressed in future studies.Third, we could not always distinguish between patients admitted through the emergency room and those transferred from other hospitals, which could have altered the MCV values.Finally, macrocytosis is indicated by a high MCV on the CBC test and is confirmed by peripheral blood smear analysis.Thus, further study by peripheral blood smear is required.

Conclusion
This study revealed that the prevalence of initial macrocytosis was 16.3% in major trauma patients and that it was independently associated with 30-day mortality along with other contributing factors such as age ( ≥ 65 years), transfusions within 4 h, hypotension, and anemia.Further studies involving peripheral blood smear and biological mechanisms are warranted to confirm that the increased MCV value reflects the enlargement of erythrocytes.

Figure 2 .
Figure 2. Cox regression proportional hazards analysis.Hazard ratios for significant variables for 30-day mortality (HR hazard ratio, CI class interval).

Table 2 .
Comparison of clinical parameters between the initial macrocytosis and normocytosis groups.Hb hemoglobin, RBC red blood cell, MCV mean corpuscular volume, Transfusion_4 transfusion within 4 h, Transfusion_24 transfusion within 24 h, pRBC_4 count of packed red blood cell within 4 h, FFP_4 count of fresh frozen plasma within 4 h, LoICU length of intensive care unit stay.